The present invention relates in general to thyristors and other four-layer devices, and more particularly to the fabrication of thyristor devices having low breakover voltages.
Thyristors, SIDACtor(copyright) overvoltage devices and other four-layer devices are commonly used to provide overvoltage protection to circuits requiring the same. The SIDACtor(copyright) overvoltage devices are two-terminal thyristors that have bidirectional current carrying capability. Such devices are obtainable from Teccor Electronics at many different breakover voltage values. When utilized in conjunction with telephone lines, for example, of the type in which 220 volt ringing signals are carried, a 250 volt breakover voltage SIDACtor(copyright) overvoltage device can be utilized to allow normal operation of the telephone line, but operate at 250 volts, or greater, in response to lightning strikes or power line crosses to thereby safely clamp the line to a very low voltage. This type of a device provides high surge current capabilities for protecting equipment from damage due to the extraneous voltages that may be coupled to the telephone line. Five-pin telephone line protection modules utilizing these high voltage devices have typically been commercially available.
Many telephone circuits and equipment operate on a xe2x88x9248 volt supply voltage. To that end, SIDACtor(copyright) overvoltage devices that operate at a nominal 64 volts are often utilized to protect such type of circuits. A nominally operating 30 volt SIDACtor(copyright) device can be advantageously utilized to protect many 24 volt circuits, such as fire alarm and other systems, that are susceptible to extraneous voltages. It can be appreciated that the lines that generally require protection from damage due to extraneous voltages are often in environments where energy from lightning strikes can be induced into the lines, where high voltage AC circuits are in close proximity thereto, and for a host of other reasons.
While low-voltage digital lines, such as those driven by 5-volt TTL drivers are extensively employed in computerized and other equipment, such lines have not yet found a large application in outside installations. However, in view of the fact that computer networks and communications are increasing at a substantial rate, such low-voltage lines are being used in environments where overvoltage protection is required. Such overvoltage protection need not be due solely to lightning and power line crosses, but can be due to other standard voltages that are commonly found in indoor equipment.
It is well known in the thyristor and SIDACtor(copyright) overvoltage device field that the impurity level of a semiconductor wafer can be adjusted to thereby achieve a desired breakover voltage. It is commonly known that lightly-doped silicon substrates are characterized by high breakover voltages. As the doping or impurity level of the substrate is increased, the breakover voltage is reduced. It is also well known that the impurity level of a semiconductor material is inversely proportional to the resistivity thereof.
It has also been found that the use of buried regions in the semiconductor substrate facilitates the operational characteristics of a SIDACtor(copyright) overvoltage device. See, for example, U.S. Pat. No. 5,479,031 by Webb. Referring to FIG. 1, if the SIDACtor(copyright) device is constructed so as to have an N-type emitter 18, a P-type base 16 and an N-type substrate 12 or mid-region, a heavily doped P-type buried region 14 can be implanted between the base region 16 and the silicon substrate 12 to thereby reduce the breakover voltage. Important advantages are achieved when the buried region 14 is directly beneath the emitter region 18, with the base region 16 material therebetween. Without significantly changing the impurity levels of the emitter 18, base 16 and substrate 12, the breakover voltage can be changed by simply changing the impurity level of the buried region 14. Moreover, in achieving breakover voltages from 250 volts down to 64 volts, the buried region need only be more heavily doped. In like manner, to achieve 30-volt breakover voltage devices, the buried region is required to be even more heavily doped.
As the impurity level of the buried region 14 increases, the junctions 20-26 formed between the buried region 14 and the base region 16 are displaced upwardly toward the emitter region 18. Indeed, as the doping level of the buried region 14 increases, the distance between the buried region-base junction 20 and the base-emitter junction becomes smaller and smaller. The reason for this is that the junction 20 is formed at a location in the semiconductor material where the donor states of one impurity are cancelled by the acceptor states of the opposite impurity. Stated another way, the junction of two semiconductor materials exists where the impurity concentration of one region is equal to the impurity concentration of the other region. The formation of a low breakover voltage SIDACtor(copyright) overvoltage device is not an elementary task.
It has been found that to fabricate nominal 10-volt breakover voltage SIDACtor(copyright) devices, the impurity level of the buried region must be so high that the buried region can often be effectively short circuited to the emitter region. In any event, even after fine tuning the processes so as to prevent short circuiting between the buried region and the emitter, the yield of workable devices is low, and thus such devices become costly.
Another problem attendant with upward migration of the junction of the buried region is that the base region under the emitter becomes thinner. The distance in the base region between the emitter junction and the buried region junction defines, in part, a holding current (Ih) parameter. The holding current is that current required to maintain an on-state of the device. A thinner base region adversely affects the ability to control a desired holding current.
Various other attempts have been made to make low breakover voltage thyristors. One endeavor involves a semiconductor design in which the breakover voltage occurs at the surface of the device. In other words, the concentration of the impurities at the surface of the device is controlled to achieve a low breakdown voltage.
Standard twisted pair telephone lines are protected by various circuits from hazardous voltages and currents that may be imposed on the lines. It is a standard practice to provide primary protection by the use of five-pin protection modules in the central offices and other high density conductor applications. Such modules have a standard pin configuration so that the modules of many different suppliers can be inserted into the same type of socket.
The basic protection to telephone lines includes primary protection modules and secondary protection modules. The primary protection module provides overvoltage protection against lightning strikes and power line crosses to the telephone lines. Such primary protectors may include gas discharge tubes and other semiconductor devices that can withstand high voltages. Secondary protection circuits often include semiconductor devices, resistors, positive temperature coefficient devices and other components to provide lower voltage protection to the customer side equipment. A family of overvoltage protection SIDACtor(copyright) devices providing the secondary protection is available from Teccor Electronics, Irving, Tex. The primary protection module is effective to limit the hazardous line voltages to approximately 300 volts. The secondary protection circuits, for example in line cards, provide additional protection to the telephone lines at levels below 300 volts.
While numerous five-pin primary protection modules are commercially available to provide primary protection, there is a limited selection of five-pin secondary protection modules providing secondary protection.
Recent changes in regulatory requirements suggest the use of DC isolation as well as overvoltage protection in secondary protection circuits of certain types of equipment. This imposes additional constraints not currently satisfied by currently available devices and circuits.
From the foregoing, it can be seen that a need exists for a method and technique to fabricate low breakover voltage thyristor devices. Another need exists for a technique to fabricate low voltage thyristor devices where the breakover voltage is independent of the holding current. Yet another need exists for a thyristor device which can be reliably made with high yields, thereby reducing the cost of the devices. Another need exists for a five-pin communication line protection module for use with low voltage communication lines.
In accordance with the principles and concepts of the invention, there is disclosed a technique for fabricating low-voltage thyristor devices, which technique overcomes the disadvantages and shortcomings of the prior art.
In accordance with an edge-fired embodiment of the invention, the buried region is laterally offset from the emitter region. The upward movement of the buried region junction as a function of the impurity level does not thereby interfere or otherwise become too close to the emitter junction. In addition, because of the lateral displacement of the buried region from the emitter, the base region underlying the emitter does not vary in thickness as a function of the location of the buried region junction. This essentially makes the breakover voltage independent of the holding current value of the device.
In accordance with another feature of the invention, a deep base is provided to thereby make the mid-region of the substrate thinner. The mid-region of the substrate functions in the four-layer device as a base of one of the regenerative-connected transistors of the thyristor device. With a thinner transistor base, the gain of the device is higher, thereby allowing the thyristor device to remain in an on state with a lower holding current.
In another embodiment, a four-layer thyristor is fabricated utilizing a pair of spaced-apart emitters with the buried region disposed therebetween.
In yet another embodiment, a low voltage thyristor device is formed as a center fired device in which the buried region is formed offset from the emitter, but generally centered in the chip. This arrangement not only allows an increased device current to flow, but also facilitates assembly of the packaged device. By placing the buried region in the center of the semiconductor chip and utilizing two symmetrically oriented metal contacts, the chip self centers itself to a lead frame when reflow soldered thereto.
In yet another embodiment, a five-pin communication line protection module utilizes the low-voltage thyristor to provide low voltage line protection to other circuits, such as data systems.